For example, a thermal type flow rate sensor is configured to be able to measure a flow rate using the fact that between a flow rate and the quantity of heat removed from an object, a predetermined relationship exists. Specifically, as a sensor mechanism, two electrical resistive elements are provided in a flow path, voltages are controlled so as to keep the temperatures of the respective electrical resistive elements constant, and from the values of the respective voltages at the time, a sensor output value x having a predetermined relationship with a flow rate is calculated (see Patent Literature 1).
In order to convert the sensor output value x to a flow rate value y, a flow rate characteristic function with a sensor output value x as an input and a flow rate value y as an output is used. The flow rate characteristic function is affected by the specific difference of the flow rate sensor from others. In other words, a flow rate characteristic function is a function specific to each flow rate sensor. Accordingly, for example, when directly using a standard flow rate characteristic function f(x) obtained by averaging the sensor characteristics of multiple flow rate sensors, an error occurs between a calculated flow rate value y and the flow rate value y of actually flowing fluid. For this reason, a correction is made in order to bring the standard flow rate characteristic function f(x) close to an actual flow rate characteristic function F(x) of an actual flow rate sensor.
For example, as illustrated in FIG. 7(a), the standard flow rate characteristic function f(x) is such that linearity is substantially kept from the zero point to the span point.
As illustrated in FIG. 7(b) next, a sensor output value x obtained when flowing standard gas such as N2 at a prescribed flow rate value in an actual sensor is measured as a sample value. As the prescribed flow rate value, for example, a span flow rate value obtained from the standard flow rate characteristic function f(x) at the span point is set. Then, by inputting the sample value to the standard flow rate characteristic function f(x), a calculated flow rate value is calculated, and further by multiplying the standard flow rate characteristic function f(x) by a function correction value n obtained by dividing the prescribed flow rate value by the calculated flow rate value, a final flow rate characteristic function h(x) having an adjusted slope is calculated. The final flow rate characteristic function h(x) calculated in this manner is used in place of the standard flow rate characteristic function f(x). Accordingly, the flow rate sensor can be corrected so as to inevitably output the span flow rate value when the sensor output value x is the same as the sample value.
However, when making a correction as described above even though the linearity of an actual flow rate characteristic function F(x) is poor as illustrated in FIG. 7(c), the final flow rate characteristic function h(x) outputs a flow rate value y smaller than an actual flow rate value y from the zero point to the span point. That is, a flow rate error occurs except for the span flow rate value.